A split-type sliding bearing for a crankshaft in an internal combustion engine is comprised of a pair of semi-cylindrical halves which are fitted in a housing half which is a part of an engine block and a housing half which is a bearing cap, so as to be formed into a cylindrical body. A bearing retaining bore in the split-type bearing housing is machined in a single working process so as to have a true-circle shape in the condition that the pair of housing halves are mated with each other by fastening bolts, before the pair of semi-cylindrical bearings are fitted therein.
In recent years, an engine block made of an aluminum alloy is used in general in order to reduce the weight of the internal combustion engine in an internal combustion engine for a passenger vehicle. In this case, one of the housing halves of an usual split-type bearing housing for a crankshaft is a part of the engine block made of an aluminum alloy, and the other one is a bearing cap made of iron alloy.
On the other hand, semi-cylindrical bearings of a usual split-type sliding bearing for a crankshaft is composed of a steel back plate and a bearing alloy layer. The outer surface of the split-type sliding bearing comprised of a pair of semi-cylindrical bearings has a circumferential length which is longer than that of the inner surface of the split-type bearing housing by a predetermined length. With this dimensional relationship, when the pair of semi-cylindrical bearings is fitted in the split-type bearing housing, a circumferential compressive stress is induced in the pair of semi-cylindrical bearings, and at the same time, a radial stress is also induced therein. Accordingly, the pair of semi-cylindrical bearings is fixed to the inner surface of the split-housing body in close contact, and the split-type bearing housing is elastically deformed so as to radially expand, resulting in an increase of its inner diameter.
The inner surface of the split-type sliding bearing comprised of the pair of semi-cylindrical halves and the outer surface of the crankshaft define therebetween a bearing clearance for feeding lubrication oil. If the bearing clearance is excessively large, the crankshaft would have a play, and as a result, vibration and noise would be caused from the internal combustion engine.
The inner diameter of the bearing retaining bore in the split-type bearing housing and the outer diameter of the crankshaft inevitably incur machining errors during manufacturing thereof. Thus, the space between the split-type bearing housing and the crankshaft becomes uneven. In order to appropriately set the bearing clearance between the inner surface of the split-type bearing housing and the outer surface of the crankshaft, a split-type sliding bearing having a suitable wall thickness is selected so as to restrain unevenness of the bearing clearance.
As stated above, however, when the split-type sliding bearing is incorporated in the split-type bearing housing, the inner diameter of the split-type bearing housing is expanded and deformed in the radial direction thereof. When the above-mentioned expansive deformation is generated, the bearing clearance is increased from a designed bearing clearance which is determined by the inner diameter of the bearing retaining bore in the split-type bearing housing, the outer diameter of the crankshaft and wall thickness of the semi-cylindrical bearings, by a degree of expansive deformation of the semi-cylindrical bearing housing. The above-mentioned expansive deformation also incurs unevenness.
JP-A-10-175131 discuses the above-mentioned unevenness, and proposes reducing the unevenness among sizes of the space between the split-type bearing housing and the crankshaft, which is caused by the expansive deformation of the split-type bearing housing, by a selective combination between the circumferential length of the split-type sliding bearing and the inner diameter of the bearing retaining bore of the split-type bearing housing in order to reduce the unevenness of the bearing clearance.
In order to reduce the weight of the internal combustion engine, the stiffness of the split-type bearing housing for a crankshaft has been conventionally decreased. An engine block made of an aluminum alloy is broadly used in order to reduce the weight.
Explanation will be made below of the relationship between the split-type bearing housing and the split-type sliding bearing comprised of a pair of semi-cylindrical bearings which are incorporated in the split-type bearing housings, referring to FIGS. 9 and 10.
The split-type bearing housing shown in FIG. 9 is comprised of a pair of housing halves, that is, a housing half 02 which is a part of an engine block and a housing half 03 which is a bearing cap (made of e.g. an iron alloy). A bearing retaining bore 05, 06 having a true-circular cross-sectional shape is formed by machining at a room temperature in the condition that the housing half 03 is mated to the housing half 02 by fastening bolts 04. Thereafter, during assembling of the bearing device, the bolts 04 are removed from the bearing housing 01, and then semi-cylindrical bearings 07, 08 which constitute the split-type sliding bearing are fitted along the inner surface 05, 06 of the bearing retaining bore. Then, the housing half 03 is mated to the housing half 02 by again fastening the bolts 04 (see FIG. 10).
With this configuration, the inner diameter (05) of the housing half 02 having a higher thermal expansion coefficient and made of an aluminum alloy becomes smaller than the inner diameter (06) of the housing half having a lower thermal expansion coefficient and made of an iron alloy due to a difference in thermal expansion coefficient between the housing half 02 made of an aluminum alloy and the housing half 03 made of an iron alloy during starting of the internal combustion engine in a cold district, even though no stepped difference is present at the inner surface 05, 06 of the bearing retaining bore 05, 06 between the abutting end faces of the pair of housing halves 02, 03, when the housing halves 02, 03 are mated with each other at a room temperature after the semi-cylindrical bearings 07, 08 are fitted in the bearing retaining bore (05, 06) between the housing halves, since the machining of the bearing retaining bore (05, 06) are carried out at a room temperature. Accordingly, a stepped difference (see reference mark “G” in FIG. 10) is caused due to a difference between degrees of thermal expansion, at the inner diameter of the bearing retaining bore (05, 06) between the circumferential end faces of the housing halves 02, 03 of the split-type bearing housing, resulting in occurrence of a stepped difference at the inner surface of the bearing between the abutting end faces of the semi-cylindrical bearings 07, 08 fitted between the housing halves 02, 03.
In recent years, an oil pump has been miniaturized in an internal combustion engine, and accordingly, the supply quantity of lubrication oil onto the inner surface of the sliding bearing for a crankshaft has been decreased. Accordingly, the bearing clearance between the inner surface of the sliding bearing for a crankshaft and the outer surface of the crankshaft is set to be small in order to reduce the amount of leakage of lubrication oil from the bearing clearance.
Furthermore, since the housing half on the engine block side made of an aluminum alloy has a thermal expansion coefficient which is larger than that of the crankshaft made of an iron alloy, the bearing clearance between the inner diameter of the pair of semi-cylindrical bearings fitted in the split-type bearing housing, and the crankshaft becomes extremely small during operation at a low temperature. Thus, if the stepped difference is caused between the circumferential end faces of the pair of semi-cylindrical bearings at the inner surface of the bearing, the ratio of the area of the stepped difference with respect to the cross-sectional area of the passage of the lubrication oil becomes larger during starting of the internal combustion engine in a cold district, in comparison with a conventional bearing clearance which has been set to be larger, resulting in occurrence of a wiping event which blocks the flow of the lubrication oil, and accordingly, the amount of leakage of lubrication oil is increased. Furthermore, the problem of inferior supply of lubrication oil onto the sliding surface of the bearing has been raised more and more.
Although JP-A-10-175131 proposes measures for comparatively decreasing the bearing clearance between the inner surface of the sliding bearing and the outer surface of the shaft in order to enhance the silence of the internal combustion engine, there is discussed no consideration as to the problem of occurrence of the stepped difference at the inner surface of the bearing between the abutting end faces of the pair of semi-cylindrical bearings which are fitted between the pair of housing halves constituting the split-type bearing housing and respectively having different thermal expansion coefficients, when the temperature of the split-type bearing housing is lowered in a cold district.